This application claims priority to an application entitled xe2x80x9cData Communication Device and Method in CDMA Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Jan. 7, 1999 and assigned Ser. No. 99-873, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a CDMA communication system, and in particular, to a device and method for assembling and de-assembling or decomposing a logical transmission unit (LTU) used for efficient data transmission in a radio environment.
2. Description of the Related Art
In general, CDMA-2000 systems have a supplemental channel that operates at a high data rate. Although the following description will use the example of a supplemental channel in a CDMA-2000 system, the present invention can apply to any system having two or more cyclic redundancy code (CRC) fields in a transmission frame, as will become clear in the description below. FIG. 1 shows the protocol layer structure of the supplemental channel in the CDMA-2000 system. Although the CDMA-2000 system has several different types of upper layer entities, FIG. 1 shows an RLP (Radio Link Protocol) layer 111 for the upper layer entity, by way of example. The RLP layer 111 assembles data received from an upper layer into an RLP frame. A multiplex sublayer 112 receives the RLP frame(s) from the RLP layer 111 and assembles RLP frame into MuxPDUs. (Multiplex sublayer Protocol Data Units). A supplemental channel physical layer element 113 receives the MuxPDUs from the multiplex sublayer 112 and assembles the MuxPDUs into a supplemental channel (SCH) frame, which is transmitted over the physical channel.
The supplemental channel physical layer element 113 is the physical layer of the supplemental channel and refers to the hardware structure of the supplemental channel. The supplemental channel physical layer element 113 receives data transmitted from the multiplex sublayer 112, fills the payload of a SCH frame with the received data, generates CRC (Cyclic Redundancy Code) bits, and then attaches the CRC bits and a tail of 8 zero (0) bits to the end of the SCH frame. Once assembled, the SCH frame is encoded by the supplemental channel physical layer element 113, and then transmitted to the receiving side.
In FIG. 1, the multiplex sublayer 112 receives transmission data from an upper layer entity, in this case RLP, and fills MuxPDUs with the received data. The multiplex sublayer 112 writes specific information in the MuxPDU header so that the multiplex sublayer on the receiving side will know to which upper layer entity the payload of the received MuxPDU should be transferred. When it is not possible to fill the SCH frame payload with any more MuxPDUs, a particular type of MuxPDU is used to fill the remaining space in the physical frame. Herein, this particular type of MuxPDU will be referred to as a fill-MuxPDU or padding (bits).
The above process of assembling MuxPDUs from RLP frames, and then assembling SCH frames from MuxPDUs is performed during transmission. Below, the process of de-assembling or decomposing SCH frames on the receiving end is described.
First, upon receipt of a SCH frame, the supplemental channel element 113 performs decoding on the received SCH frame. After decoding, the supplemental channel element 113 calculates the CRC bits for the received payload, and then compares the calculated CRC bits with the CRC bits received with the SCH frame. As noted above, the received CRC bits were calculated and transmitted by the supplemental channel element on the transmission side. If the CRC bits are identical to each other, the supplemental channel element 113 provides the multiplex sublayer 112 with the payload of the received supplemental channel frame along with information indicating that the payload passed the CRC check. If the CRC bits are different from each other, the supplemental channel element 113 provides the multiplex sublayer 112 with the payload of the received supplemental channel frame along with information indicating that the payload failed the CRC check. Upon receipt of information indicating the payload passed the CRC check, the multiplex sublayer 112 examines the provided payload from the beginning to separate the MuxPDUs. On the other hand, upon receipt of information indicating the payload failed the CRC check, the multiplexer sublayer 112 discards the provided payload and informs each upper layer entity that an error frame has been received.
FIG. 2 shows the structure of the SCH frame which is transmitted over the supplemental channel. Referring to FIG. 2, the SCH frame is comprised of a payload, a 16-bit physical layer CRC, and 8 tail bits indicating termination of the SCH frame for encoding. As indicated above when the transmission procedure was described, the payload is filled with several MuxPDUs and the remainder is filled with fill-MuxPDUs (padding bits).
In any radio transmission system, burst errors occur in the data stream. In the CDMA-2000 system, where the SCH frame is transmitted using a convolutional encoding method, burst errors occur from place to place inside the payload of the SCH frame, because of the long length of the SCH frame. In this case, because there are many MuxPDUs in the SCH frame, there will be some MuxPDUs that have no errors as well as the one or more MuxPDUs that have errors. It is much more efficient to separate the error-less MuxPDUs instead of discarding them, and transfer all of them to the upper layer as correctly received data.
In order to exploit this efficiency, the MuxPDUs are grouped into larger units, called logical transmission units (LTUs). The LTU has a specific size and has a CRC field for indicating whether or not the LTU has an error. Therefore, when the physical layer CRC check on the SCH frame shows an error, the multiplex sublayer 112 won""t discard the entire payload. Instead, the multiplex sublayer 112 separates out the individual LTUs in the SCH frame payload, and performs an LTU CRC check on each individual LTU. When there is an error, the multiplex sublayer 112 discards the LTU. Otherwise, when there is no error, the multiplex sublayer 112 separates out the MuxPDUs included in the LTU and provides the data to the upper layers.
Using this LTU function, each LTU has a CRC field, which is additionally filled into the SCH frame payload. This means that the room in the payload for MuxPDUs is reduced by the size of the CRC field. Therefore, there is a need for a method of maximizing the number of MuxPDUs for transmission while maintaining the existing MuxPDU size. In addition, it is proposed that a 16-bit, or 2-byte, CRC field should be used for byte alignment in arranging the LTU in the payload of the physical channel frame. The preferred embodiment of the present invention proposes a 12-bit LTU CRC field so that more MuxPDUs can be placed in the payload of the SCH frame. In addition, the bytes in the MuxPDUs retain byte alignment, even though the CRC field is 1 and a xc2xd bytes long.
It is, therefore, an object of the present invention to provide a device and method for effectively arranging a CRC field in a CDMA communication system.
It is another object of the present invention to provide an LTU assembling device and method which retains byte alignment without reducing the size of a MuxPDU, thereby increasing efficiency, in a communication system.
It is another object of the present invention is to provide a byte alignment method using at least two logical CRC fields in one physical frame.
To achieve the above and other objects, there is provided a method for arranging CRC fields within logical transmission units, (LTUs) in a CDMA (Code Division Multiple Access) communication system having a physical channel frame comprised of a plurality of LTUs arranged in line, followed by the physical layer CRC field and tail bits. Each LTU has a payload containing data and a LTU CRC field for error correction of the data. In the LTU CRC arranging method, the LTU CRC field included in odd-numbered LTUs is placed in the back of the LTU and the CRC field included in even-numbered LTUs is placed in the front of the LTU. Because each odd-numbered LTU is followed by an even-numbered LTU, the two CRC fields form a three byte boundary between adjacent LTUs, resulting in the retention of byte alignment.